As known in the art, valve actuation in an internal combustion engine controls the production of positive power. During positive power, intake valves may be opened to admit fuel and air into a cylinder for combustion. One or more exhaust valves may be opened to allow combustion gas to escape from the cylinder. Intake, exhaust, and/or auxiliary valves may also be controlled to provide auxiliary valve events, such as (but not limited to) compression-release (CR) engine braking, bleeder engine braking, exhaust gas recirculation (EGR), internal exhaust gas recirculation (IEGR), brake gas recirculation (BGR) as well as so-called variable valve timing (VVT) events such as early exhaust valve opening (EEVO), late intake valve opening (LIVO), etc.
As noted, engine valve actuation also may be used to produce engine braking and exhaust gas recirculation when the engine is not being used to produce positive power. During engine braking, one or more exhaust valves may be selectively opened to convert, at least temporarily, the engine into an air compressor. In doing so, the engine develops retarding horsepower to help slow a vehicle down. This can provide the operator with increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle.
One method of adjusting valve timing and lift, particularly in the context of engine braking, has been to incorporate a lost motion component in a valve train linkage between the valve and a valve actuation motion source. In the context of internal combustion engines, lost motion is a term applied to a class of technical solutions for modifying the valve motion dictated by a valve actuation motion source with a variable length mechanical, hydraulic or other linkage assembly. In a lost motion system the valve actuation motion source may provide the maximum dwell (time) and greatest lift motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage between the valve to be opened and the valve actuation motion source to subtract or “lose” part or all of the motion imparted from the valve actuation motion source to the valve. This variable length system, or lost motion system may, when expanded fully, transmit all of the available motion to the valve and when contracted fully transmit none or a minimum amount of the available motion to the valve.
An example of such a valve actuation system 100 comprising a lost motion component is shown schematically in FIG. 1. In particular, the system 100 illustrated in FIG. 1 is representative of a portion of the teachings found in U.S. Patent Application Publication No. 2010/0319657 (“the '657 Publication”), the teachings of which are incorporated herein by this reference. As shown, the valve actuation system 100 includes a valve actuation motion source 110 operatively connected to a rocker arm 120. The rocker arm 200 is operatively connected to a lost motion component 130 that, in turn, is operatively connected to one or more engine valves 140 that may comprise one or more exhaust valves, intake valves, or auxiliary valves. The valve actuation motion source 110 is configured to provide opening and closing motions that are applied to the rocker arm 120. The lost motion component 130 may be selectively controlled such that all or a portion of the motion from the valve actuation motion source 110 is transferred or not transferred through the rocker arm 120 to the engine valve(s) 140. The lost motion component 130 may also be adapted to modify the amount and timing of the motion transferred to the engine valve(s) 140 in accordance with operation of a controller 150. As known in the art, valve actuation motion source 110 may comprise any combination of valve train elements, including, but not limited to, one or more: cams, push tubes or pushrods, tappets or their equivalents. As known in the art, the valve actuation motion source 110 may be dedicated to providing exhaust motions, intake motions, auxiliary motions or a combination of exhaust or intake motions together with auxiliary motions.
The controller 150 may comprise any electronic (e.g., a microprocessor, microcontroller, digital signal processor, co-processor or the like or combinations thereof capable of executing stored instructions, or programmable logic arrays or the like, as embodied, for example, in an engine control unit (ECU)) or mechanical device for causing all or a portion of the motion from the valve actuation motion source 110 to be transferred, or not transferred, through the rocker arm 120 to the engine valve(s) 140. For example, the controller 150 may control a switched device (e.g., a solenoid supply valve) to selectively supply hydraulic fluid to the rocker arm 120. Alternatively, or additionally, the controller 150 may be coupled to one or more sensors (not shown) that provide data used by the controller 150 to determine how to control the switched device(s). Engine valve events may be optimized at a plurality of engine operating conditions (e.g., speeds, loads, temperatures, pressures, positional information, etc.) based upon information collected by the controller 150 via such sensors.
As further shown in FIG. 1, the rocker arm 120 is supplied with hydraulic fluid from a hydraulic fluid supply 160. Where the lost motion component 130 is hydraulically actuated, the hydraulic fluid provided by the hydraulic fluid supply 160 (as dictated, for example, by the controller 150) flows through the rocker arm 120. In the so-called bridge brake implementation taught in the '657 Publication, the lost motion component 130 resides in a valve bridge (not shown in FIG. 1) and comprises a check valve that permits the one-way flow of fluid into the lost motion component 130.
In such systems, the supply of the necessary hydraulic fluid is of critical importance to the successful operation of the valve actuation system 100. FIG. 2 illustrates an embodiment of known exhaust valve motions employed to perform compression release braking as function of valve lift (vertical axis) relative to crankshaft angle (horizontal axis), including a main exhaust valve event 202, a compression release valve event 204 and a BGR valve event 206. As shown in FIG. 2, the supply of the necessary hydraulic fluid in prior art systems (including the systems taught in the '657 Publication) occurs between the end of the main exhaust valve event 202 and the beginning of the BGR valve event 206, i.e., an event requiring actuation of the lost motion component 130. However, when operating at high engine speeds, the illustrated refill period can be very short. As a result, the pressure and flow of the hydraulic fluid may not be adequate to actuate the lost motion component 130, which in turn may result in loss of performance or high loading on the valve train.
To address this situation, the '657 Publication describes a system in which an accumulator 170 is provided in the valve bridge, which accumulator 170 is configured to harvest hydraulic fluid periodically discharged by the lost motion component 130. Consequently, the accumulator 170 is configured to reside downstream of the check valve residing in the lost motion component 130. During subsequent actuations of the lost motion component 130, i.e., during the refill period illustrated in FIG. 2, the accumulated hydraulic fluid is used to supplement the supply of hydraulic fluid otherwise provided by the rocker arm 120 to the lost motion component 130.
While the above-described system in the '657 Publication represents a welcome advancement of the art, still further solutions may prove advantageous.